And to implement templates proposal computer class. This class instance should be created from ExamplePythonCompletionProposalComputer.createTemplateProposalComputer() method.
ExamplePythonTemplateCompletionProcessor.java

Templates support has one requirement IDLTKUILanguageToolkit class should return not null source viewer configuration from createSourceViewerConfiguration() method.
We need to overwrite ui language toolkit declared in second part and return correct source viewer configuration: